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Thursday, December 24, 2009

Loop Antenna

A year or so ago, I started experimenting with 30m Loop Antenna.

Not knowing exactly what I was doing I started with Grey PVC Conduit and Electrical Box Fittings. I used two 10 foot sections bent into a circle with a square box fitting at the bottom and a tee pull-box at the top. The loop is about 19 feet in circumference (a little was cut off to remove the bell ends). Bending was done last summer in the hot sun. I have made two of these and have done different experiments on each - and, have learned something new at each revision.

Top of the loop Tuning 4-47pF Cap

The Bottom Box, with Toroid Core for Matching

(sorry, my old camera can not take a good photo for here)

Somewhere I had read that a single loop antenna had a very low impedance feed point 180 deg from the top or from the tuning capacitor. I used 12 AWG as the element inside of the conduit, looping through the conduit from the top, down to the toroid and back up to the top (that was fun!?).

Loop Schematic
(Now measured at 18.70 feet Circumference)

30m Loop As-Built

Experientially, I found a 4x2 turn toroid would provide a close match to my 75 ohm feed source, I used my AIM-4170B to check the feed performance. Also, during the experiment I found that 4-47pF cap would resonate the loop at 30m (10.140mHz). With a little tweaking of the toroid turn count and ratio, I should be able to get a low 1:1 SWR.

When I did all of this, I only had QRSS station and Grabber running, I did not have WSPR to make proper antenna comparison measurement. But. it did work well for both QRP QRSS Transmit and Grabber operation.

Recently, I have been reading Joachim's blog, where he describes the DCTL (Distributed Capacitance Twisted Loop) antenna. Well, my antenna looks somewhat like a DCTL antenna, only it has a lump capacitor at the top. I have started thinking I should rewire it with 300 ohm twin lead and driving it with my toroid transformer.

The next experiment, is going to use 300 ohm Twin Lead for the element. My concern is not the actual resonate frequency, only it's feed point behaviour, I can fix the resonate frequency later by change the loop size or a tuning cap.

My goal is to build a loop that is simple and easy, and that will be easy to keep in tune.

UPDATE1:

I have measured the wire length or the inside of the conduit at 5.7m (18.70 feet) and calculated 1.81m (5.95 feet) Diameter.